(function () {
   var o ={a:1}
   Object.defineProperty(o,'b',{
     get () {
     return this.a
    },
     set (newVal) {
      this.a = newVal
     },
     configurable: true // b 是否可以被再次配置
   })
   console.log(o.b)
   o.b = 2
   console.log(o.b)
})()